By dilating coronary stenoses, percutaneous transluminal coronary angioplasty (PTCA) can relieve angina pectoris and improve exercise tolerance and left ventricular function. However, restenosis occurs in 20-30% of dilated stenoses within 3-6 months following PTCA making it necessary to restrict patient activities, resume antianginal medications, repeat PTCA, or perform coronary artery bypass surgery. Balloon dilatation of the atherosclerotic lesion damages the endothelium, intima, and media of the artery. This may lead to restenosis via platelet deposition, mural thrombus formation, and intimal proliferation by mechanisms that appear similar to those causing aortocoronary vein graft (ACVG) occlusions. We have demonstrated that dipyridamole plus aspirin therapy can supporess these mechanisms of ACVG occlusion in the animal model, prolong a shortened platelet survival in patients with coronary artery disease, and reduce ACVG occlusions in patients both early and later after the operation. Thus, a trial of these drugs in patients undergoing PTCA is a logical and necessary step in our efforts to reduce the major shortcoming of the initially successful PTCA therapy, namely the high rate of restenosis. A prospective, double-blind, randomized, platelet-inhibitor, drug trial with dipyridamole plus aspirin versus placebo will be conducted in patients undergoing PTCA to evaluate drug effectiveness in reducing the incidence of restenosis. The primary end points to be compared in the treated and control groups by coronary angiography performed 6 months after PTCA are (1) the percent of patients with successful PTCA who have restenosis of the dilated lesion and (2) the degree of change in the dilated lesion from immediately after PTCA to 6 months later. Determination of the degree of angiographic stenosis before PTCA, immediately after PTCA, and 6 months after PTCA, will be done by 2 methods with which we have had previous experience: (1) consensus reading of arteriograms by 2 groups of 2 observers on 2 occasions (method of Brensike et al) to visually estimate the maximal percent narrowing in arterial diameter and (2) light pen outlining of arterial lesions on a digitizing tablet interfaced with a computer. Secondary end points include new coronary events (myocardia infarction, death, and need for aortocoronary bypass graft operation or PTCA).